The present invention relates to a method and apparatus for controlling an inverter of the load commutation type.
Inverters of the load commutation type are often used as a power source for an induction heating apparatus, such as an induction furnace, an induction heater or the like, as, for example, shown in FIG. 1. As illustrated, the input or DC terminals of a load commutation type inverter 1 is connected through a DC reactor 2 to a controllable DC power source 3. The output or AC terminals of the inverter 1 are connected to a load 4 comprising an induction furnace or heater 4a with a parallel capacitor 4c for phase advancement. The load 4 is shown in the form of an equivalent tank circuit.
The inverter 1 is of a current type, as is often the case where an inverter is used as a power source for an induction heating apparatus. The inverter 1 is operated at a frequency corresponding or equal to the resonant frequency of the load tank circuit 4, which itself varies for various reasons, e.g., when the material to be heated is charged into the heating apparatus, or discharged, or when the temperature of the material to be heated varies.
The inverter 1 is provided with a constant margin time control circuit 5, which receives a signal obtained by a voltage detector 6 detecting the voltage between the terminals of the load 4 and supplied therefrom through a rectifier 7, whose output is shown in FIG. 2A. The control circuit 5 is of a type which performs a constant margin time control in which the time margin for turn-off of each thyristor of the respective one of the arms of the inverter is maintained constant. The control circuit 5 may, for example, have an internal construction as shown in FIG. 3. As illustrated, it comprises a zero detector 21 which receives the output 7a (FIG. 2A) of the rectifier 7 and produces a pulsative output 21a when the output 7a is zero. A sawtooth wave generator 22 produces a sawtooth waveform voltage 22a which is reduced to and starts at a zero level every time the output 7a becomes zero, and rises at a constant gradient. A subtractor 23 subtracts a predetermined constant value from the output 22a of the sawtooth wave generator 22. A latch circuit 24 samples the value of the output 23a of the subtractor 23 when the output 21a is produced, and stores the value obtained by sampling. Upon each sampling, the latch circuit 24 produces a pulse 24b which is used to reset the sawtooth wave generator 22, i.e., to bring the output 22a to the zero level. A comparator 25 compares the outputs 22a and 24a and produces a pulse 5a (FIG. 2B) when they are identical. The output 5a of the comparator 25 constitutes the output of the control circuit 5. The time interval between the production of the pulse 5a and the end of each half cycle of the voltage across the load 4, i.e., the time point at which the output 7a subsequently becomes zero is kept substantially constant provided that the frequency of the voltage across the load 4 does not vary rapidly, and hence constant margin time control is thereby achieved. An example of such a circuitry for achieving the constant margin time control is disclosed in Japanese Patent Publication No. 4102/1981.
The output signal of the control circuit 5 is delivered, through a distributor 8, as gate signals, to the thyristors forming respective arms of the inverter 1. More particularly, the distributor 8 supplies the incoming pulses to the thyristors of a first pair of arms 1a and 1d, during one half cycle of the voltage across the load, and to the thyristors of a second pair of arms 1b and 1c during the next half cycle, and alternately, in a similar manner, to the thyristors of the first pair, and the second pair during the subsequent half cycles. In this way, the frequency of the inverter 1 is automatically adjusted in accordance with the power factor of the load 4.
A disadvantage of the above-described system is that no-load start or no-load operation is not possible, and the voltage across the load 4 rises, when the load is too light, so high a value that the thyristors of the inverter 1 are damaged.